Process for preparation of cadmium sulfide crystals



United States.

This invention relates to the preparation of crystalline cadmium sulfide adapted for use in solar batteries, photo- Cells, X-ray and gamma ray detection, infrared windows, and for other purposes. The principal objective of this invention has been to provide a method for producing large single crystals of cadmium sulfide from which individual pieces may be Cut of such sizes as are required for use.

Cadmium sulfide has no known melting point at ordinary pressures; rather, it sublimes and goes directly into the vapor phase from the solid phase. Because of the nonexistence of the liquid phase of this compound at ordinary pressures, previous methods for the preparation of crystalline cadmium sulfide have been confined to vapor phase processes. One such method, for example, is that of Frerichs, as described in Naturwissenschaften, Vol. 33 C1946), p. 281, wherein hydrogen sulfide and cadmium are reacted at high temperature. Alternatively, Reynolds and Czyzak, in Phys. Rev., vol. 79 (1950), p. 543, have proposed that cadmium sulfide powder be sublimed and subsequently recrystallized directly from the vapor phase at low pressure.

The products of such methods are polycrystalline, i.e., the solid mass resulting from the procedure is comprised of four, five or a greater multiplicity of individual crystals all enmeshed rather than a single crystal and the electrical resistivity of the solid mass, which is a measure of the quality or usability, varies over a wide range throughout the crystal aggregation. In order to obtain useful products from such a mass or to secure therefrom the portions having the most desirable properties, it has been necessary to cut the mass into slices, examine the slices individually, and reject those which are of inferior quality. On this account, a large amount of waste is produced. Heretofore, there has been no known method for producing large single crystals of uniform size. Neither has there been any means of producing single crystals having small quantities of other substances, called dopants, distributed uniformly therein through the incorporation of which the electrical properties of the crystals may be improved or accurately controlled.

This invention is predicated upon the discovery and determination that single crystals of cadmium sulfide may be produced by progressive, controlled freezing of a melt of cadmium sulfide. More specifically, it has been discovered that single crystals, which may be of large size if desired but which in any event are of a much more homogeneous and uniform nature than any products known heretofore, may be produced by melting densilied cadmium sulfide powder under high pressure and cooling the molten cadmium sulfide slowly and progressively from one end to permit the desired single crystal growth. Thus, in the practice of this invention the product is crystallized from molten rather than from gaseous cadmium sulfide. The method of Crystallizing from the liquid phase inherently permits the utilization of certain techniques whereby the crystallization process may be subjected to exacting control measures so as to obtain crystals of maximum homogeneity and quality.

As previously indicated, cadmium sulfide at atmospheric pressure sublimes when heated. However, when the compound is heated under highpressure, say of the Crucible.

Patented Apr. 30, 1963 'ice order of atmospheres, it does melt, but even at such pressure, there is a tendency for the vapor phase to form, partially by sublimation from the unmelted solid phase material and partially by evaporation from the material already melted. Since the process of this invention is based on the discovery that single crystal growth can be obtained from a melt, it is desirable to eliminate vaporization of the melt to as great a degree as is possible, because Vaporization in effect uses up the raw material from which the crystal is made and particularly because deposits or inclusions resulting from direct vapor to solid state transition impair crystal quality and homogeneity. Furthermore, at high temperatures cadmium sulfide tends to dissociate; that is, it will break down into its constituent elements in accordance with the reversible reaction CdSCd-l-S the vapor dissociating more readily than either the solid or the liquid. The presence of either of the dissociation products in the melt also impairs the quality of crystals formed from the melt, in that the dissociation products are frozen into the crystal as impurities. However, it has been determined that the adverse effects from either source can be avoided by maintaining a high pressure during the slow cooling period.

In particular, we have found that if the melting and crystallization procedures are conducted in the presence of a chemically inert or nonreactive gas maintained at high pressure, both the tendency of the cadmium sulfide to vaporize and its tendency to dissociate are effectively retarded, at least to a degree at which they are chemically and functionally acceptable and that a single crystal of remarkable size and uniformity Can be produced which displays superior electrical characteristics. Argon is a suitable blanketing gas, although it is only representative of the class of inert gases all of which possess the required electronic completeness. In an argon atmosphere at a gauge pressure of approximately 100() p.s.i., for instance, complete melting takes place at a temperature of about l500 C. To minimize vaporization and/or thermal decomposition of the cadmium sulfide, it is recommended that melting be carried out as rapidly as possible, for example in ten minutes or less.

The cooling of the melt to obtain the crystal growth may be accomplished in any manner enabling the melt to be cooled slowly and progressively, for example, in a period of at least about three hours. During cooling, the solid-liquid interface is a substantially planar surface which moves slowly through the melt proceeding, say, from one end of the melt to the other end. Two means for progressively freezing the melt in the required manner are described here for examples and are not intended to be by Way of limitation.

(l) The cadmium sulfide is melted in a Crucible of graphite or other material inert to t-he melt and blanketing gas located within a surrounding heater. The heater is preferably of the electrically operated, resistively heated type and is maintained at a constant heat throughout the entire process. The Crucible is held in proximity to the heater by extensible supporting means by which it may be gradually withdrawn or removed from the heater. When the cadmium sulfide charge is being melted, the supporting means are so adjusted as to hold the crucible in an attitude of close approach to the heater until the Charge is entirely molten. Then, holding iconstant the power input of the heater, the Crucible is very gradually withdrawn from the heater by the supporting means, that portion of the melt Ifarthest from the heater receiving less heat and therefore `becoming relatively cooler so that the melt begins freezing from the farthest removed end of the This -lirst frozen portion' of the melt apparently acts as a seed crystal, which grows in mass as the Crucible continues to be removed from the heat source. In other words, as solidification proceeds slowly and progressively from one end of the melt to the other, a large desirable single crystal continues. The ra-te at which the Crucible is withdrawn from the heater governs the rate of crystal growth in the Crucible and the quality of the crystal grown as well, slower growth -as a rule of thumb resulting in higher quality crystals. The invention provides a degree of Control over crystal growth and quality not known in vapor phase procedures.

(2) Alternatively, the heater, Crucible, and insulation are so designed with respect to one another that when the heater is in operation a thermal gradient is set up along some dimension of the heater. The heater is so designed that its temperature will ydiffer at different stations along, say, its length, whereby one end of the heater and, consequently, one end of the melt, are at a relatively higher temperature than the other end of the heater and melt, the temperature gradually decreasing at stations successively further removed from the hot end. The power input of this apparatus is variable and to melt the sulfide, the power source is adjusted to bring the temperature of that end of .the heater which is relatively the coolest to `a temperature above the melting point of the sulfide, thus causing all the sulfide contained in the Crucible to melt. As the temperature of the Crucible and Charge within it correspond generally or proportionally to the temperature of the immediately adjacent portion of the heater, control over the temperature of the heater affords a directly proportional Imeasure of control over the temperature of the melt, but it should be understood that this relationship need not be lineal. Once melting is Completed, crystallization is begun by `slowly reducing the power input 4to the heater so that the coolest zone of the melt is brought to its freezing temperature, whereupon it solidilies. As power input is further reduced, progressively successive portions freeze.

An important aspect of this invention' resides in the discovery that an incident of controlled crystallization is the regulated infiuence it in turn exerts on the quantities and distribution in the crystal of dopants. These may be added to the charge of cadmium sulfide in the Crucible in measured quantities, and go into solution in the sulfide when the latter is melted. It has been found that the presence inthe crystal of precise quantities of, for example, the sul-fides of gallium or indium will produce a crystal exhibiting unique electrical behavior. Whereas, in vapor-phase-grown crystals dopants, if present, were inevitably distributed throughout the body of the crystal in an irregular or heterogeneous pattern, crystals produced in accordance with this invention display high regularity of dopant distribution. The product of the present method thus has a higher homogeneity than the vapor phase product, independen-tly of its crystal singularity. For example, a representative crystal produced in accordance with ths invention displays a prole of electrical resistivity, measured at stations along its longitudinal axis, which varies between the limits of .48 and 260 ohmcentimeters, while the range of a comparable vapor-phase grown crystal extends between the limits of .007 and l li ohm-centimeters. Furthermore, it is characteristic of the method that the dopants are selectively segregated, so that the producer has control over the quantity of dopants present per volume of crystal, as well as over the distribution thereof. This results from the fact that the solute, or dopant, concentration in the freezing solid cadmium sulfide differs from that in lthe liquid; the difference reflects the nature of the equilibrium between liquid and solid cadmium sulfide phases. The solid-liquid interface, travelling through the melt, produces a calculable redistribution of the solutes in the charge. If the particular dopant in' question lowers the melting point of the cadmium sulfide solvent (i.e., the melted material), its concentration in the freezing solid will be lower than in the liquid, and hence the dopant will be rejected by the `reezing solid and will accumulate in the liquid, to be frozen in at the last-solidifying end of the melt, which may be cut off to leave the remainder having uniform predetermined characteristics. The precise behavior of selected dopants depends on their concentration and their nature; in general, we have found that lead and copper are strongly rejected by the crystals and go to the last frozen part of the melt, while zinc, indium, manganese, and cobalt stay in the crystal and do not segregate.

The practice of the process of this invention is best further explained in connection with the accompanying drawing. The drawing is a schematic View in section through an :apparatus `representative of the type in which the production of the crystals may take place. A heavywalled pressure vessel, or bomb, is indicated at 1. It is designed to have sumcient strength to withstand internal pressures of about one hundred atmospheres under use conditions for prolonged periods of time and may suitably be constructed as a forging of steel or other metal. This vessel contains an internal cavity 2, in which resides a Crucible 3 on a standard 4 for containing the cadmium sulfide charge 5. The vessel is internally equipped with a heating element 6 capable of heating the Crucible and sulfide within it to a temperature sufficiently high to cause the complete melting of the sulfide. The temperature required for this is approximately l500 C. The heating element may consist, for example, of a graphite helix or a split graphite tube. The :heater electrodes may be water-cooled molybdenum rods with Igraphite studs Connecting .the electrodes and heater. `In the arrangement shown, the resistively operated heater 6 is supplied with power from an' external power source 7 connected to it through leads 8, a switch 9, and a variable rheostat 10 for controlling the power input, but other conventional power supply circuit and control means may be used. The bomb is jacketed for water cooling to help prevent shell rupture, the jacketing not being shown in the drawing.

The unusually severe conditions which exist in the bomb cavity during the production of a crystal necessitate the use of insulative means; at a temperature of l500 C. and a pressure of atmospheres there is a certain probability, not negligible, that an uninsulated steel bomb would rupture. To maintain the -bomb itself at a relatively low temperature, thereby mitigating the severity of its operating conditions, the insulation is preferably located adjacent the cavity walls as at 11 rather than at the outside walls of the bomb. This concurrently reduces the power demands of the heater, because less heat is transferred to the bomb. It is preferable that the cavity insulation be nonporous to prevent heat losses to the bomb wall by convection currents as well as by conduction. Such losses would otherwise result at the high temperatures maintained in the cavity during operation by reason of the fact that the gas contained therein under high pressure and at a high temperature has a correspondingly high kinetic energy and is very heat conductive. Thus, the use of a conventional porous insulation results in excessive heat loses by the convective circulation of the gas through the interconnected porosities of the insulations. However, we have found that a highly effective insulation for this purpose is provided by fused stabilized zirconia, of which the chemical formula is ZrO2. This material has a low heat conductivity, is sufficiently dense to minimize convection losses, and is nonreactive and stable in the atmosphere present in the bomb at the high temperatures required. A pressure line 12 leads to the cavity through the bomb walls and insulation through which the cavity may be evacuated and also through which an inert or nonreactive gas may be introduced into the bomb to bring the cavity conditions to one of high pressure. It has been determined that argon gas is Well suited for use as the ambient atmosphere but the use of other inert gases such as helium or neon is feasible.

Under stabilized operating conditions a temperature gradient exists between the top and bottom of the crucible. This gradient is such as to cause the temperature at the lower or bottom end of the crucible to be lower than the temperature at the upper end of the crucible. An alternative means of establishing the same conditions is to provide mechanical means by which the entire crucible is movable from within the heater in such a manner as to cause freezing and crystallization first to occur at one end of the melt. As an illustration, a vertical piston may be used to move the crucible downwardly and out of the heater, thus rst removing the lower end of the crucible to a region of lower temperature so that freezing is induced in that portion of the melt. Again, these examples are by way of illustration only as other methods of heat control within the crucible may be conceived by one skilled in the art.

Typical practice of the invention, wherein the crystallization of cadmium sulfide is carried out in a bomb in which a thermal gradient is established by the heater, is described as follows:

ICadmium sulfide powder is densified by heating it to 900 C. in an evacuated furnace. Upon removal from the furnace, the material is crushed to particles of approximately one quarter inch size. These are introduced into a tapered cylindrical Crucible, which may be made of graphite. Dopants, if desired, may be added as raw material charge at this point. The particular nature of these will, of course, depend on the intended use of the crystal and may vary within Wide limits. The crucible 3 is supported on the standard 4 positioning it in the heater 6 in the pressure furnace. The furnace is assembled, and evacuated through the gas line 12 to remove all traces of air. It is then lled with argon gas to a gauge pressure of at least 500 and preferably 1,000 p.s.i. During subsequent heating of the sealed bomb, this pressure will increase to about 1500 p.s.i. due to the increase of kinetic energy of the gas with temperature, and the pressure is maintained at substantially the latter value during the remainder of the operation. Cooling water is run through the electrodes and power to the resistively operated heater elements is turned on at the switch 9. The power input through the rheostat is adjusted so that a high temperature is maintained in the crucible until the cadmium sulde is entirely melted. The melting phase of the operation is carried out as rapidly as possible to minimize vaporization and/ or thermal decomposition of the cadmium sulfide. When the charge is entirely melted, progressive cooling of the melt is begun. This is carried out by reducing, at a very slow rate, the power input to the heater. The thermal gradient which exists along the length of the Crucible causes the bottom of the melt to first reach freezing temperature. As the power input is further decreased, the solid-liquid interface 13V is moved slowly upwardly into the melt. This slowly progressive freezing not only encourages conditions for good crystal growth but rejects partly decomposed cadmium sulfide towards the liquid melt. It is preferable that the cooling period extend to at least three hours, at the end of which time a single crystal will be found in the crucible. At the end of cooling, the crystallized cadmium sulfide, still retained within the bomb, is annealed at operating pressure at approximately 100 C. for a period of four hours. rIlhis relieves the internal stresses and strains formed in the single crystal during crystallization. At the end of the annealing period, the pressure inside the bomb is very slowly reduced to zero to avoid cracking the crystal or setting up further stresses within it. The furnace is then opened and the crystal is removed from the crucible.

It is apparent that by adjustabil-ity of power input rate-of-cooling control is afforded. For temperature measurement and control, a series of thermocouples may be located within or close to the Crucible and connected to indicators outside the bom enabling the operator to adjust power input as required to bring about t-he desired temperature conditions. Or, after the design of the elements has been established, temperature-power input curves for the stabilized system could be empirically determined by which the operator might know the temperature simply by reference to a set of tables. In a bomb which has mechanical means for removing the crucible from the heater practice of the invention would be similar to that already described with the exception that melt temperature would be related to the relative degree of withdrawal of the Crucible rather than to power input, the power input remaining constant throughout the crystallizing process.

Thus, the present invention provides means enabling single crystals of cadmium sulfide of large size to be grown (a) which display constant size and shape, (b) in which the concentration of solute impurities is subject to accurate control, and (c) in which the solutes are homogeneously distributed.

While there has been disclosed in the above description what is deemed to be the most practical and efcient embodiment of the invention, it should be understood that the invention is not limited to such embodiment, as there might be changes made without departing from the principle of the invention as comprehended within the scope of the accompanying claims.

Having described our invention, what we claim is:

l. A process for preparing `a large single crystal of cadmium sulde, said process comprising,

densifying cadmium sulfide powder by heating it to a temperature of substantially 900 C. in a vacuum, crushing the densified cadmium sulfide, melting said densied sulfide at a pressure of at least 500 p.s.i. in an inert gas, at a temperature of at least about 1500 C.,

said melting being effected rapidly in a period of less than about ten minutes so that sublimation and dissociation of said sulde are reduced,

and maintaining the molten sulfide under a pressure above about 500 p.s.i. while cooling the melt over a period of at least about three hours,

said cooling being conducted in such manner that a substantially planar solid-liquid interface moves progressively through the melt in a single direction.

2. The process of claim l wherein said melting is effected at a pressure between about 1000 and 1500 p.s.i.

3. The process of claim 2 wherein said melting is conducted at a temperature of about 1500 C.

4. The process of claim 2 wherein said cooling period is greater than 3 hours.

5. A process for preparing a large single crystal of cadmium sulfide, said process comprising,

melting cadmium sulfide powder in a gas to which said `sulfide is inert, at a pressure of at least about 500 p.s.i. and at a temperature of at least about 1500 C., said melting being effected rapidly in a period of less than about 10 minutes so that sublimation and dissociation of said sulfide are reduced, and' maintaining said sulfide under a pressure above about 500 p.s.i. while freezing the molten sulfide over a period of at least about 3 hours,

said freezing being effected by a gradual reduction of the temperature at a pre-selected point in the melt until freezing is initiated at that point, and thereafter continuing to reduce the temperature in such manner that the solid-liquid interface advances through the melt in a given direction from the pre-selected point.

6. The process of claim 5 wherein a desired solute impurity is admixed with said powder and is distributed homogeneously through said crystal by said melting and freezing steps.

7. The process of claim 5 wherein said melting is effected at a pressure of `between about 1000 and 1500 p.s.i. and wherein said freezing is conducted at a pressure of between 1000 and 1500 p.s.i.

8. The process of claim 5 wherein the molten sulfide is frozen over a period longer than three hours.

References Cited in the le of this patent UNITED STATES PATENTS 2,019,259 Fisher Aug. 31, 1937 2,220,117 OBrien Nov. 5, 1940 2,674,520 Sobek Apr. 6, 1954 2,754,180 Horton July 10, 1956 2,843,914 Koury July 22, 1958 2,958,932 Goercke Nov. 8, 1960 8 FOREIGN PATENTS 695,936 Great Britain Aug. 19, 1953 OTHER REFERENCES Pfann: Zone Melting, John Wiley & Sons, New York, April 18, 1958, pages 158 and 1,61.

Physical Review, vol. 97, No. 6, page 1526, March 15, 1955.

Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. IV, Longmans, Green and Co., London (1923), pages 589, 594, and 597. 

1. A PROCESS FOR PREPARING A LARGE SINGLE CRYSTAL OF CADMIUM SULFIDE, SAID PROCESS COMPRISING, DENSIFYING CADMIUM SULFIDE POWDER BY HEATING IT TO A TEMPERATURE OF SUBSTANTIALLY 900*C. IN A VACUUM, CRUSHING THE DENSIFIED CADMIUM SULFIDE, MELTING SAID DENSIFIED SULFIDE AT A PRESSURE OF AT LEAST 500 P.S.I. IN AN INERT GAS, AT A TEMPERATURE OF AT LEAST ABOUT 1500*C., SAID MELTING BEING EFFECTED RAPIDLY IN A PERIOD OF LESS THAN ABOUT TEN MINUTES SO THAT SUBLIMATION AND DISSOCIATION OF SAID SULFIDE ARE REDUCED, AND MAINTAINING THE MOLTEN SULFIDE UNDER A PRESSURE ABOVE ABOUT 500 P.S.I. WHILE COOLING THE MELT OVER A PERIOD OF AT LEAST ABOUT THREE HOURS, SAID COOLING BEING CONDUCTED IN SUCH MANNER THAT A SUBSTANTIALLY PLANAR SOLID-LIQUID INTERFACE MOVES PROGRESSIVELY THROUGH THE MELT IN A SINGLE DIRECTION. 